Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Including Purchased Gas Adjustments, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 63.4	$ 72.8	$ 149.3
Regulatory assets, non current	655.1	669.8	716.6
Total Regulatory Assets	718.5	742.6	865.9
Regulatory liabilities, current	37.0	35.7	21.5
Regulatory liabilities, non current	363.4	354.6	335.1
Total Regulatory Liabilities	400.4	390.3	356.6
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	29.7	30.7	76.7
Regulatory assets, non current	427.6	441.1	484.1
Total Regulatory Assets	457.3	471.8	560.8
Regulatory liabilities, current	20.6	16.7	2.7
Regulatory liabilities, non current	279.7	274.9	241.2
Total Regulatory Liabilities	300.3	291.6	243.9
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	18.8	26.2	58.0
Regulatory assets, non current	200.2	201.5	197.4
Total Regulatory Assets	219.0	227.7	255.4
Regulatory liabilities, current	10.2	7.6	11.3
Regulatory liabilities, non current	30.6	31.3	39.0
Total Regulatory Liabilities	40.8	38.9	50.3
Pension and Postretirement Benefit Costs
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.2	30.2	43.0
Regulatory assets, non current	350.0	364.9	394.8
Pension and Postretirement Benefit Costs | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	21.9	21.9	34.9
Regulatory assets, non current	279.8	292.5	315.8
Pension and Postretirement Benefit Costs | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.3	7.3	7.2
Regulatory assets, non current	63.0	64.8	70.8
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	3.0	8.2	77.9
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current		1.0	38.5
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	1.6	6.4	39.4
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.2	34.4	28.4
Regulatory assets, non current	38.4	42.4	54.8
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.8	7.8	3.3
Regulatory assets, non current	17.5	21.4	25.2
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	9.9	12.5	11.4
Regulatory assets, non current	3.2	3.3	2.7
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	99.1	96.3	113.1
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	96.7	94.4	113.1
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	134.0	133.4	123.9
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	134.0	133.4	123.9
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	33.6	32.8	30.0
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	33.6	32.8	30.0
Rate Stabilization and Equalization ("RSE") Adjustment
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.5	0.0	1.0
Rate Stabilization and Equalization ("RSE") Adjustment | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.5	0.0	1.0
Refundable Negative Salvage
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	4.6	5.2	7.9
Regulatory liabilities, non current		0.0	3.8
Refundable Negative Salvage | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	4.6	5.2	7.9
Regulatory liabilities, non current		0.0	3.9
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	6.1	2.9	1.0
Regulatory liabilities, non current	16.7	4.7	6.5
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	5.8	1.9	0.0
Regulatory liabilities, non current	16.7	4.7	6.5
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	23.8	27.6	11.6
Regulatory liabilities, non current	90.4	80.2	34.2
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	14.8	14.8	2.7
Regulatory liabilities, non current	75.4	69.3	23.5
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	3.1	2.4	2.4
Regulatory liabilities, non current	3.5	3.5	3.6
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	169.8	178.3	177.4
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	152.4	161.1	159.2
Pension and Postretirement Benefit Costs
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	27.1	27.8	31.5
Pension and Postretirement Benefit Costs | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	27.1	27.8	31.5
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	59.4	63.6	81.7
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 35.2	$ 39.8	$ 52.0